Optical data storage technology is capable of storing data, such as audio or video information, at very high densities, and has many applications in both consumer and professional fields. As is well known, such optical data storage technology is used in the reading and writing of compact disks (CD), as well as in the reading and writing of the more recently developed digital video disks (DVD). The introduction of the DVD has resulted in an increase in data storage capacity of more than ten times as compared with the CD, this increase having been brought about by a combination of tighter system tolerances and a decrease in the laser wavelength used to read or write information on the disk, for example from 780 nm to about 650 nm. Further increases in data storage capacity are realisable if the laser wavelength is further reduced to the blue and ultraviolet (UV) parts of the spectrum.
There are two groups of semiconductor compounds and alloys which are capable of emitting light in the blue and UV parts of the spectrum. These are the Group II-VI semiconductor materials denoted generally as (Zn, Mg) (S, Se), where such notation indicates the different compounds formed by combining either Zinc (Zn) or Magnesium (Mg) with Sulphur (S) or Selenium (Se), and Group III-V semiconductor materials from the alloy system denoted by (Al, Ga, In)N, where such notation indicates the alloys formed by combining Aluminium (Al), Gallium (Ga) or Indium (In) with Nitrogen (N)). The former group is most suited to emission in the blue-green part of the spectrum, whilst alloys and compounds of the latter group are particularly suited to emission in a wavelength range spanning orange, through blue to UV.
Progress in the development of Group II-VI semiconductor materials for use in light-emitting devices has resulted in the announcement of 100 hours cw operation of a blue-green laser diode (LD) by S. Taniguchi et al, Electron. Letters, 32, 552 (1996). Whilst this is an impressive achievement, progress in the development of Group III-V semiconductor materials has been even more significant over the last few years. In 1994, the successful realisation of a (InGa)N/(AlGa)N double heterostructure, high brightness blue light-emitting diode was reported by S. Nakamura et al, Appl. Phys. Lett., 64, 1687 (1994). This was followed in 1995 by an announcement of the successful realisation of high brightness blue and violet light-emitting diodes by S. Nakamura et al, Appl. Phys. Lett., 67, 1868 (1995), based on the use of (InGa)N quantum wells (QW) in the active region of the diode. In 1996, pulsed operation at room temperature of an (InGa)N QW laser diode was reported by S. Nakamura et al, Jpn. J. Appl. Phys., 35, L74 (1996). Recently the pulsed operation of an (InGa)N QW laser diode has been announced in Toshiba Corporation, Press Release, 11th Sep. 1996, and the cw operation at room temperature of a 412 nm (InGa)N MQW laser diode has been announced by S. Nakamura et al, late news paper at the IEEE-LEOS Annual Meeting, Boston, November 1996.
These reported results have led to considerable interest being shown in the growth of III-Nitride semiconductor materials and the fabrication of light-emitting diodes and laser diodes based on such materials. Such materials have mainly been produced by the method of epitaxial growth known as Metal Organic Chemical Vapour Deposition (MOCVD) which is also known as Metal Organic Vapour Phase Epitaxy (MOVPE). However it should be noted that such materials can also be produced by the epitaxial growth method known as Molecular Beam Epitaxy (MBE) as reported by, for example, R. J. Molnar et al, Appl. Phys. Lett., 66, 268 (1995). This is approach has resulted in the achievement of p-type doping and weak electroluminescence (EL) at room temperature from both GaN homojunction light-emitting diodes and (InGa)N/GaN heterojunction light-emitting diodes. Whilst the results obtained from the semiconductor materials produced by the MBE growth method are currently inferior to the results obtained from semiconductor materials produced by the MOCVD growth method, there are potential advantages in producing such semiconductor materials using the MBE growth method due to the fact that the temperature difference between the growth temperatures of (InGa)N and GaN (or (AlGa)N) is smaller when the MBE growth method is used than when the MOCVD growth method is used, as will be described in more detail below.
A significant problem in the epitaxial growth of III-Nitride semiconductor materials is the hetero-epitaxial nature of the growth process. GaN semiconductor material is only available in non-commercially viable pieces of a few millimeter in dimension so that most growth of GaN is carried out on a Sapphire substrate. Alternative substrate materials have been tried, such as Silicon Carbide (SiC), various oxides such as Lithium Gallate, and Spinel. Without exception, GaN is lattice mismatched from these substrates. For example, the lattice constant of Sapphire is approximately 12.5% larger than that of GaN, and this leads to the generation of many defects at the interface between the GaN and Sapphire. However it appears that GaN is much more fault tolerant than other Group III-V semiconductor materials, and GaN-based light-emitting diodes can operate successfully for extended periods even where there are approximately 10.sup.10 cm.sup.-2 defects in the material. Additionally the differential thermal expansion between the epilayer and the substrate can lead to the generation of dislocations in the layers of the device if the strain energy is not accommodated elastically.
Until commercially viable GaN substrates become available, such problems of hetero-epitaxy and the resulting dislocations that it introduces seem unavoidable. Meanwhile one empirical solution is to grow a sufficiently thick layer of GaN (on a suitable buffer layer) until the layer becomes fully relaxed. Further layers can then be deposited epitaxially onto the layer with the GaN lattice constant. It is also likely that many of the dislocations that are introduced at the substrate-buffer interface will have turned over, and will not therefore penetrate through the whole of the GaN layer if it is sufficiently thick.
A further problem in the growth of III-Nitride semiconductor materials is a function of the design of the light-emitting diode structure used. FIG. 1 diagrammatically illustrates the light-emitting diode structure used by S. Nakamura et al, Appl. Phys. Lett., 64, 1687 (1994) as reported above. This structure was produced using a MOCVD growth method. A GaN buffer layer 2 of a thickness of about 300 .ANG. was grown on a Sapphire substrate 1 at about 510.degree. C., followed by a n-doped GaN contact layer 3 of a thickness of about 4 .mu., a n-doped (AlGa)N cladding layer 4 of a thickness of about 1.5 .mu.m, and a Zn-doped (InGa)N active layer 5 of a thickness of about 500 .ANG.. After growth of the active layer 5, p-doped cladding and contact layers 6 and 7 of (AlGa)N and GaN were grown to thicknesses of about 0.15 .mu.m and 0.5 .mu.m respectively, and finally a n-type electrode 8 and p-type electrode 9 were evaporated onto the n-doped contact layer 3 and the p-doped contact layer 7.
Furthermore FIG. 2 shows a graph of the variation of the lattice constant a against the band gap energy for the quaternary system (Al, Ga, In) N. In the light-emitting diode structure of FIG. 1, the In mole fraction in the active layer 5 of the device is approximately 0.06 whilst the Al mole fraction in the surrounding cladding layers 4 and 6 is approximately 0.15. It will be appreciated from FIG. 2 that none of the (AlGa)N cladding layers 4 and 6 and the (InGa)N active layer 5 are latticed matched with GaN or each other, the strain being approximately .+-.1% relative to GaN. If some of the resulting strain is not accommodated elastically, then the energy is released in the form of dislocations in the active region of the light-emitting diode or laser diode. Such dislocations would clearly have a deleterious effect upon the efficiency of operation of the device.
A further complication results from the need to grow the (InGa)N active layer 5 at a substrate temperature which is approximately 200-300.degree. C. lower than the temperature used to grow either the GaN contact layer 7 or the (AlGa)N cladding layer 6, that is at a temperature of 700-800.degree. C. as compared with a temperature of 1020.degree. C. for growth of the layers 6 and 7, due to the re-evaporation of Indium from the growing surface at elevated temperatures. This re-evaporation effect can be significant as shown by C. -K. Sun et al, Appl. Phys. Lett., 69, 1936 (1996) where a graded (InGa)N layer was produced by ramping the growth temperature from 760.degree. C. to 700.degree. C. during evaporation of the Indium. The resulting variation in the Indium mole fraction across the layer as a function of the distance from the interface is shown in the graph of 2 of FIG. 3.
U.S. Pat. No. 5,476,811 discloses a method of manufacturing a laser diode having a GRIN-SCH structure and comprising a GaAs active layer sandwiched between two graded layers of composition Al.sub.x Ga.sub.1-x As where the Al constituency is varied across the layer in accordance with an accurately controlled compositional profile. The AlGaAs graded layers are produced by metal-organic molecular beam epitaxy while changing the temperature of the substrate so that the compositional parameter x is decreased during growth of a first graded layer prior to growth of the active layer, and the parameter x is increased during growth of the second graded layer on the active layer. Such epitaxial growth of graded layers of defined thickness using a defined crystal orientation serves to form optical confinement layers between the GaAs active region and the AlGaAs cladding regions providing an energy band structure confining the carriers to the active region. However such devices do not suffer from deleterious effects due to dislocations caused by lattice mismatching between the active layer and the cladding layers. Furthermore the graded layers disclosed in U.S. Pat. No. 5,476,811 would not be suitable for use in compensating for lattice mismatching in a GaN heterostructure in view of their constituency and thickness which are specifically adapted to their intended function as confinement layers.
It is an object of the invention to provide a method of producing III-nitride optoelectronic semiconductor device, such as a light-emitting semiconductor device, which enables the results of lattice mismatching and the resulting deleterious dislocations to be reduced.
According to the present invention, there is provided A III-Nitride optoelectronic semiconductor device having an active region of a III-Nitride semiconductor material which is lattice mismatched with a further III-Nitride semiconductor material of one or more cladding regions, the device comprising a substrate and, formed sequentially on the substrate, a first contact region of one doping type, a first cladding region of said one doping type, an active region, a second cladding region of the opposite doping type, and a second contact region of said opposite doping type, wherein, in order to compensate for the lattice mismatching between the active region and one or both of the cladding regions, a graded layer is interposed between the active region and one or both of the cladding regions which is such that one side of the graded layer is lattice matched with the adjacent active region and the other side of the graded layer is lattice matched with the adjacent cladding region and the graded layer has a constituency, for example a Group III constituency, which is graded from said one side to said other side of the graded layer.
The effect of the graded layer is to reduce the strain at the interface between the regions separated by the layer which suffer from significant lattice mismatching, and to thereby minimise the possibility of deleterious dislocations being introduced at the interface which could then propagate through the active region of the device. By removing or reducing such dislocations, the efficiency of operation of the device is increased. However it is not necessary for the device to be symmetric about the active region, and the cladding regions in particular may be of different compositions and/or thicknesses. Where graded layers are provided on both sides of the active region, these may also be of different compositions and/or thicknesses.
Preferably the graded layer between the active region and one or both of the cladding regions has a first constituent which is graded across the layer in one direction and a second constituent which is graded across the layer in the opposite direction. Furthermore the thickness of the or each graded layer is preferably in the range of 20 to 400 .ANG., most preferably in the range of 30 to 300 .ANG..
Preferably one or both of the cladding regions comprises a III-Nitride semiconductor material which is lattice mismatched with a III-Nitride semiconductor material of the adjacent contact region, and a further graded layer is interposed between one or both of the cladding regions and the adjacent contact region such that one side of the further graded layer is lattice matched with the adjacent cladding region and the other side of the further graded layer is lattice matched with the adjacent contact region and the further graded layer has a constituency, for example a Group III constituency, which is graded from said one side to said other side of the further graded layer.
In one embodiment the active region comprises a quantum well or multiquantum well disposed between two guide regions, and an additional graded layer is interposed between one or both of the guide regions and the well. This embodiment is applicable to a laser diode.
Each of the contact regions, the cladding regions and the active region preferably incorporate gallium as a constituent, and most preferably the active region incorporates indium whereas the cladding regions incorporate aluminium.
The invention also provides a method of growing a III-Nitride optoelectronic semiconductor device having an active region of a III-Nitride semiconductor material which is lattice mismatched with a further III-Nitride semiconductor material of one or more cladding regions, the device comprising a substrate and, formed sequentially on the substrate, a first contact region of one doping type, a first cladding region of said one doping type, an active region, a second cladding region of the opposite doping type, and a second contact region of said opposite doping type, the method comprising the steps of successively growing the first contact region, the first cladding region, the active region, the second cladding region and the second contact region on the substrate, and, between the growth of the active region and the growth of one or both of the cladding regions, growing a graded layer which is such that one side of the graded layer is lattice matched with the adjacent active region and the other side of the graded layer is lattice matched with the adjacent cladding region and the graded layer has a constituency which is graded from said one side to said other side of the graded layer, in order to compensate for the lattice mismatching between the active region and one or both of the cladding regions.
The active region may be grown at a first temperature while supplying a first constituent to the substrate surface, one or both of the cladding regions may be grown at a second temperature while supplying a second constituent to the substrate surface, and the graded layer may be grown by supplying at least one of the first and second constituents to the substrate surface while the substrate temperature is changed between the first and second temperatures. In one embodiment the second temperature is greater than the first temperature, and the graded layer is grown, after the growth of the first cladding region and before the growth of the active region, by supplying at least one of the first and second constituents to the substrate surface while the substrate temperature is ramped downward. In another embodiment the second temperature is greater than the first temperature, and the graded layer is grown, after the growth of the active region and before the growth of the second cladding region, by supplying at least one of the first and second constituents to the substrate surface while the substrate temperature is ramped upward.
Furthermore the first constituent may be supplied to the substrate surface during growth of the graded layer but with the supply of the second constituent to the substrate surface being stopped during such growth. Alternatively both the first constituent and the second constituent may be supplied to the substrate surface during growth of the graded layer. In this case the second constituent may be supplied to the substrate surface at a rate which is varied monotonically during growth of the graded layer between a maximum rate at which the second constituent is supplied during growth of the cladding region and a minimum rate.
In a further embodiment one or both of the cladding regions comprises a III-Nitride semiconductor material which is lattice mismatched with a III-Nitride semiconductor material of the adjacent contact region, and, between the growth of one or both of the cladding regions and the growth of the adjacent contact region, a further graded layer is grown such that one side of the further graded layer is lattice matched with the adjacent cladding region and the other side of the further graded layer is lattice matched with the adjacent contact region and the further graded layer has a constituency which is graded from said one side to said other side of the further graded layer. In this case one or both of the cladding regions is grown while supplying a constituent to the substrate surface, and the further graded layer is grown by supplying said constituent to the substrate surface at a rate which is varied monotonically between a maximum rate at which said constituent is supplied during growth of the cladding region and a minimum rate.